In the manufacture of many articles, and in particular those comprising a metal, the article is initially formed by a casting or forging step dieting the manufacturing process. This step, and other initial forming steps, usually results in an imperfect, raw article due to pattern variation and die shift. For example, a raw shaft often has an offset that defines a seam or parting line extending along a length of the shaft which results in a quasi-elliptical or generally asymmetric cross-sectional shape. Further, a raw shaft is generally not balanced about its geometric center. In some instances, the imperfection is inconsequential, and in others, the imperfection must be removed in a subsequent refinement step during the manufacturing process.
A raw article may be refined in machining and grinding operations which remove a disposable amount of material, or cleanup stock, that is designed into the raw article. In the case where a portion of the perfected article is symmetric about an axis, for instance a shaft, these refinement operations generally require that the raw article be engaged and retained in a chuck of a lathe or other rotating machinery. Due to the asymmetrical shape of the raw shaft, prior art chucks do not accurately align the geometric center of the shaft with the rotation axis of the machinery, which is generally aligned with the rotation axis of the chuck. More specifically, in a standard four jaw chuck, normally only three of the four jaws will engage an asymmetric shaft, which results in improper alignment of the geometric center of the shaft with the rotation axis of the chuck. The disadvantages of a four jaw chuck may be overcome in part by a three jaw chuck, wherein all three jaws are likely to engage and securely retain the shaft. A three jaw chuck however does not entirely overcome the problem of inaccurate alignment of the geometric center of the shaft with the rotation axis of the chuck.
Inaccurate alignment, or chucking, in combination with the unbalanced nature of the raw shaft, may result in an unbalance load condition as the shaft is rotated about the rotation axis of the chuck. This circumstance may be detrimental to rotatable machinery, and in extreme cases, hazardous to machine operating personnel. Inaccurate alignment of a raw shaft in a chuck also frustrates the optimization of some machine operations resulting in reduced productivity and economic efficiency. For example, in the manufacture of a precision balanced rotating article, like a crankshaft, it may be advantageous to measure the imbalance of a raw, asymmetric shaft by rotating the shaft about its geometric center, and then correcting the imbalance by moving the rotation axis of the shaft in relation to the geometric center.
In view of the discussion above, there exists a demonstrated need for an advancement in the art of accurately centering an asymmetric workpiece in a chuck.
It is therefore an object of the present invention to provide a novel method and apparatus for a self compensating and geometrically centering chuck.
It is also an object of the present invention to provide a self compensating and geometrically centering chuck having two or more rotatable jaws for engaging and securely retaining an asymmetric workpiece, wherein the jaws accurately align a geometric center of the asymmetric workpiece with a rotation axis of the chuck.
It is a further object of the present invention to provide a self compensating and geometrically centering chuck that includes a rotatable ring coupled to the rotatable jaws, wherein the rotatable ring compensates for the rotation of one jaw by rotating the other jaw to accurately align the geometric center of the asymmetric workpiece with the rotation axis of the chuck.
Accordingly, the present invention is directed toward a novel method and apparatus for a self compensating and geometrically centering jaw type chuck for engaging and retaining an asymmetric workpiece, wherein a geometric center of the asymmetric workpiece is accurately aligned with a rotation axis of the chuck. The chuck includes two opposing jaws each of which is rotatable about a pivot pin extending from a radially adjustable jaw slide. Each jaw includes an outer end that is slidably and rotatably coupled to a rotatable ring symmetrically disposed about the rotation axis by a synchronizing pin having a first end fixed to the rotatable ring, and a second end, rotatable in relation to the first end, slidably engagable with the outer end of the jaw. The first ends of the synchronizing pins are disposed on opposing sides of the rotatable ring. In one embodiment, the second end of the synchronizing pin has a rectangular shape and is slidably received in a channel in the outer end of the jaw. In one embodiment, the jaws include an inner end with two teeth having workpiece engagement surfaces, and may be opened and dosed to engage and retain the asymmetric workpiece by adjusting the jaw slides along a radial of the rotation axis. The rotation of one jaw about the pivot pin rotates, in synchronism, the rotatable ring and the other jaw to accurately align the geometric center of the asymmetric workpiece with the rotation axis of the chuck. The jaw slides are radially adjusted to open and close the jaws by reciprocating a piston along the rotation axis of the chuck. The piston is coupled to the jaw slides by two opposing cranks rotatable about a crank pivot which provides a levering action to adjust the jaw slides. A passive spring assembly applies a force to the piston which biases the jaws in the dosed position to geometrically center and clamp the workpiece. The spring assembly prevents the workpiece from becoming disengaged from the jaws in the event of a power failure. The jaws are opened in response to an applied counter-force applied to the piston by pneumatic or hydraulic actuation.